"An achievement is probably already familiar. There's a goal or activity you must accomplish and a reward that you receive upon completion. You can unlock an achievement at any time, be it on a game's launch day or three years later. I guess you can say an achievement is like a promise in that sense.

"A challenge, on the other hand, is more like an opportunity - better grab it while you can! It is also comprised of a goal and a reward; however, challenges are time-bound (as in, real life time). That means you can only unlock during its eligible time window, and if you get close but don't complete the goal when it ends..."